<template>
  <div id="goTop">
    <!-- 返回顶部 -->
    <div class="goTop" v-show="goTopShow" @click="goTop">
      <i class="iconfont icon-shangjiantou goTopIcon"></i>
    </div>
  </div>
</template>
<script>
export default {
  name: 'goTop',
  data () {
    return {
      scrollTop: '',
      goTopShow: false
    }
  },
  watch: {
    scrollTop () {
      if (this.scrollTop > 500) {
        this.goTopShow = true
      } else {
        this.goTopShow = false
      }
    }
  },
  methods: {
    handleScroll () {
      this.scrollTop =
        window.pageYOffset ||
        document.documentElement.scrollTop ||
        document.body.scrollTop
      if (this.scrollTop > 500) {
        this.goTopShow = true
      }
    },
    goTop () {
      let timer = null
      const _that = this
      cancelAnimationFrame(timer)
      timer = requestAnimationFrame(function fn () {
        if (_that.scrollTop > 0) {
          _that.scrollTop -= 120
          document.body.scrollTop = document.documentElement.scrollTop =
            _that.scrollTop
          timer = requestAnimationFrame(fn)
        } else {
          cancelAnimationFrame(timer)
          _that.goTopShow = false
        }
      })
    }
  },
  mounted () {
    window.addEventListener('scroll', this.handleScroll)
  },
  destroyed () {
    window.removeEventListener('scroll', this.handleScroll)
  }
}
</script>

<style scoped>
.goTop {
  position: fixed;
  right: 40px;
  bottom: 60px;
  width: 40px;
  height: 40px;
  border-radius: 50%;
  background: #fff;
  padding: 10px;
  cursor: pointer;
  box-shadow: 0 0 6px rgba(0, 0, 0, 0.12);
}
.goTop:hover .goTopIcon {
  color: rgba(51, 153, 255, 1);
}
.goTopIcon {
  font-size: 40px;
  color: rgba(51, 153, 255, 0.8);
  text-align: center;
  margin-top:7px;
  display: block;
}
</style>
